mbcache: correctly handle 'e_referenced' bit

mbcache entries have an 'e_referenced' bit which users can set with
mb_cache_entry_touch() to indicate that an entry should be given another
pass through the LRU list before the shrinker can delete it.  However,
mb_cache_shrink() actually would, when seeing an e_referenced entry at
the front of the list (the least-recently used end), place it right at
the front of the list again.  The next iteration would then remove the
entry from the list and delete it.  Consequently, e_referenced had
essentially no effect, so ext2/ext4 xattr blocks would sometimes not be
reused as often as expected.

Fix this by making the shrinker move e_referenced entries to the back of
the list rather than the front.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

diff --git a/fs/mbcache.c b/fs/mbcache.c
index c5bd19ffa326a1c8c30dc7d89263640d0aa82739..31e54c25d16f97334a2aacc4435b3d9997a2c0f2 100644 (file)
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -286,7 +286,7 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
                                         struct mb_cache_entry, e_list);
                if (entry->e_referenced) {
                        entry->e_referenced = 0;
-                       list_move_tail(&cache->c_list, &entry->e_list);
+                       list_move_tail(&entry->e_list, &cache->c_list);
                        continue;
                }
                list_del_init(&entry->e_list);